The fight against algae

Manufacturers have a long history of trying to curb algae growth on shingles

In 1993, three of the 14 U.S. asphalt shingle manufacturers listed in the first edition of NRCA's Steep-slope Roofing Materials Guide offered algae-resistant shingles. Algae-resistant shingles filled a niche in the steep-slope roofing market, appealing to roofing contractors and consumers located in hot, humid areas of the U.S. Availability typically was restricted to the Southeastern and Gulf of Mexico coastal regions of the U.S.

Presently, nine manufacturers supply asphalt shingles to the U.S. roofing market. Eight of them offer a variety of algae-resistant shingle products that are readily available throughout the U.S. and Canada.

The growth of algae-resistant-labeled asphalt shingle sales can be interpreted as an indication the steep-slope roofing market is receiving products that provide effective protection against unsightly microbial discoloration. That, in essence, is the position of asphalt shingle manufacturers. Asphalt shingle manufacturers and algae-resistant system suppliers say this claim is based on evidence collected from long-term product exposure in weathering farms and positive feedback about product performance.

All asphalt shingle manufacturers that currently offer algae-resistant shingle products assert they possess empirical data—or have access to algae-resistant granule suppliers' long-term shingle exposure results—that support claims about the performance of their respective products. This evidence is not available to the public. In addition, no recognized standard is available for objective evaluation of long-term resistance to microbial staining of roof covering materials.

Following is what currently is known about algae resistance technology and how it is used in the asphalt roofing industry.

Blue-green algae

Nationwide surveys sponsored by Minnesota Mining and Manufacturing Co. (now 3M,™ St. Paul, Minn.) in the U.S. during the 1960s and 1980s concluded that by far the most common cause of dark stains found on asphalt shingle roof systems was colonization by hardy microorganisms from the family Gloeocapsa and, much less frequently, members of the Scytonema family.

The perpetrators belong to a group of bacteria capable of photosynthesis. Their common name, "blue-green algae," originated from observations made in environments with low light intensities. Blue-green algae can be captured from the air, found floating in fresh and sea water, and observed growing in earthbound environments.

Blue-green algae tolerate drought and heat and can survive in a dormant state during periods of winter cold. Gloeocapsa require only low-intensity light for survival, and individual cells are surrounded by a gelatinous outer sheath. When they establish a presence in locations directly exposed to sunlight, such as rooftops, the outer sheath acquires dark pigmentation, which is an adaptation to the intense ultraviolet light.

The ink-like streaks found on rooftops are accumulations of Gloeocapsa cells. In suitable environments, earthbound blue-green algae reproduce and release wind-borne spores, which spread the infestation to other surfaces, including roofs. Gloeocapsa cells produce a substance that adheres them firmly to surfaces and prevents wind or rain from dislodging established Gloeocapsa colonies.

Gloeocapsa need water and air, access to light, carbon from inorganic sources (such as carbon dioxide in air and calcium bicarbonate in limestone) and micronutrients to carry out photosynthesis and thrive.

Dew, which regularly occurs on shingles during much of their service lives, and rainwater transport the inorganic nutrients blue-green algae need to thrive. It is believed that because limestone filler commonly is used in asphalt shingles, shingles' surfaces provide hospitable space for Gloeocapsa colonization.

Figure provided by 3M Industrial Mineral Products Division, St. Paul, Minn.

Figure 1: Roof algae follow climate and geography.

A spreading nuisance

You may wonder: If asphalt shingles have been available for a century, why has the discoloration problem not gained general notoriety sooner?

In fact, the U.S. roofing industry was addressing the problem in various ways in the Southeast long before the current crop of algae-resistant shingles appeared on the market. Preference for light-colored shingles in hot, humid areas such as Florida compounded the severity of the roof discoloration problem and influenced the approach asphalt shingle manufacturers originally adopted to provide a commercial solution.

3M researchers note that booming residential construction in North America brought about conditions that led to the microbial roof discoloration problem's escalation.

The ever-expanding residential development market transformed large tracts of previously open land into urbanized areas in many parts of the U.S. and Canada. The concentration and proximity of asphalt shingle roof systems in these neighborhoods has accelerated blue-green algae's spread and, in turn, increased public awareness of the issue. As the number of affected roof systems increases, more Gloeocapsa spores are released into the air and the blue-green algae spread with greater speed and ease.

Researchers from 3M and ISP Minerals, Inc., Hagerstown, Md., have constructed maps that show the problem's severity and geographic reach (see Figures 1 and 2).

"It is a general map meant to show that blue-green algae grow where there are dew and humidity," says Rachael Gould, Ph.D., a scientist with 3M Industrial Mineral Products Division, referring to the map shown in Figure 1.

Naturally, the sight of roofs disfigured by dark streaks has not escaped homeowners' attention, and many specifically seek shingles that do not develop unsightly stains.

Asphalt shingle manufacturers have responded by publishing homeowner information bulletins. Language about algae resistance prominently is featured in manufacturers' limited shingle warranties, which likely are homeowners' first source of shingle product information. The benefits of selling algae-resistant shingles are being highlighted in marketing materials targeting asphalt shingle installers.

The blue-green algae discoloration problem has gained notoriety as—and, perhaps, because—public awareness of this issue has increased.

Figure provided by ISP Minerals Inc., Hagerstown, Md.

Figure 2: Geographical predominance of roof discoloration by algae

Metallic zinc

Early solutions to the algae discoloration problem likely were conceived from observations of stained roofs, which commonly employed galvanized steel flashings. Roof areas downslope of exposed galvanized metal were conspicuously free of the characteristic dark stains. It was found that exposed strips of zinc metal could keep a roof free of visible discoloration.

To control blue-green algae growth, zinc strips are fastened in a line near the top of a roof, such as at a ridge. Zinc oxide, zinc carbonate and other zinc salts form on the strips' surfaces upon exposure to air and rooftop moisture. Water runoff from rain and condensation that forms as dew on shingles carry these corrosion products down a roof's slope.

As long as some precipitation occurs at reasonable intervals, the roof surfaces below the strips are repeatedly treated with an antimicrobial solution of zinc ions. This wash-down protection becomes more diluted and less effective the farther it travels, so roofs with significant eave-to-ridge distance require two or more courses of zinc strips to prevent microbial growth.

Because the zinc metal is slowly but continuously sacrificed in this application, the strips have to be of sufficient thickness to provide effective antimicrobial protection for a roof's full service life. These types of products are available commercially and typically promise protection for up to 20 years.

Perhaps as a result of the zinc strips' success, the asphalt shingle industry turned to the zinc industry for the original on-shingle antimicrobial protection system. Zinc dross, in the form of granule-sized chips, was used as an antimicrobial additive in the manufacture of early algae-resistant asphalt shingles. A byproduct of commercial zinc smelting, zinc dross is a combination of zinc and impurities rejected from commercially pure zinc. Zinc dross particles were embedded on shingles with the application of surfacing granules on the exposed parts of shingles. This remained a popular algae-resistant technology through the late 1980s.

Because of its common use in galvanized flashings, zinc in metallic form had an established history of effectiveness as an algae inhibitor. In addition, its application was possible with in-place machinery and no new technologies had to be developed to easily incorporate it as part of asphalt shingle surfacing. Also, use of zinc dross additive was popular among manufacturers because the material did not tie up significant storage capacity at asphalt shingle manufacturing plants.

This early algae-resistant shingle technology was not without pitfalls. Maintaining uniform additive distribution on shingle surfaces and a consistent application rate are critical for reliable performance of algae-resistant on-shingle surfacings.

Process conditions that sometimes occur in asphalt shingle manufacturing make it difficult to maintain granule surfacing application parameters within their optimal ranges, particularly when surfacing types with different flow characteristics are used together, such as when zinc dross additive is introduced into a stream of common roof granules. Some shingle products using this algae-resistant technology placed all-zinc-dross bands of surfacing particles in the exposed section to control flow incompatibility issues and differentiate the appearance of products.

There also were other serious complications after the finished product left the production line. Zinc bloom, also known as "white rust," observed at times on and around metallic zinc exposed to stagnant moisture, was manufacturers' least favorite consequence of using zinc dross. It manifested as white stains that visibly disfigured shingles. The bloom appears as a result of oxidation and reaction with pollutants when moisture is present for extended durations but not in quantities sufficient to completely carry away zinc corrosion products before they accumulate as conspicuous bright stains around the zinc dross particles.

This problem was characteristic of zinc dross-bearing shingles that had been packaged wet and remained wet in storage. Zinc bloom also developed on shingles applied to roofs that experienced dew but did not see rain for extended time periods.

A combination of mounting complaints about blotches of zinc bloom on products and volatile zinc prices ultimately caused the U.S. asphalt shingle industry to abandon its use of zinc dross.

Early granules

During the mid-1960s, 3M introduced its original pigmented algae-resistant granules. 3M considered this product an upgrade from common white and light-colored pigmented granules intended for markets with significant demand for algae-resistant shingles, such as Florida.

GAF Materials Corp., Wayne, N.J., (whose roof granule business would later be transferred to ISP Minerals) introduced a similar granule product during the early 1970s.

Both products were designed to provide effective long-term antimicrobial protection when used as the bulk of surfacing on the exposed part of an asphalt shingle.

3M's granules used a copper-based coating, and ISP Minerals' granules were manufactured with a bimetallic copper-zinc coating. The granules were available in light colors because of the Southeast's preference for light-colored asphalt shingles.

The manufacturing processes for common pigmented roof granules and pigmented algae-resistant roof granules are similar. Both start as rock quarried from suitable mineral deposits and delivered to a crushing and coloring plant.

Following several crushing and screening steps, the base granules are ready for color coating. During the coloring process, they are coated with a slurry that consists of water, sodium silicate, clay and pigments in a rotating kiln. A firing temperature of about 900 F typically is used to transform the slurry solids into a durable ceramic granule coating. Process variables, such as the slurry composition or firing temperature, are adjusted when coatings formulated to provide algae resistance are prepared. This step can be repeated to produce multiple coating layers.

Additional coating steps may be taken to produce granule colors difficult to obtain with only one pigmented coating layer, improve granule adhesion in asphalt or act as an outer release control layer for the algae inhibitor typically incorporated into an inner granule coating layer.

This algae-resistant granule technology proved successful, and demand for it grew. Markets outside the Southeast opened up for algae-resistant shingles and preferred darker colors. As a result, light-colored algae-resistant granules were blended with common granules to obtain earth tones and other darker colors, potentially at the cost of reduced effectiveness.

Addressing growing market demand for greater variety in algae-resistant shingle colors was problematic with this algae-resistant granule technology.

Standard non-algae-resistant shingles were the bread and butter of typical asphalt roofing shingle plants. Algae-resistant shingle production was ancillary for most. Additional algae-resistant granule colors would have to be stored beside standard colors to maintain uninterrupted operation if manufacturers were to retain flexibility in production scheduling. The storage capacity of a typical asphalt roofing shingle plant was insufficient to store additional specialty colors and maintain flexible production schedules.

The situation helped suppliers of zinc dross retain a share of asphalt shingle manufacturers' business and ultimately spurred the development of the specialty algae-resistant granules currently used.

Current technology

Current-generation algae-resistant granules are top surfacing additives with controlled long-term release of algae-inhibiting copper ions. The current copper coating formulation is more potent than treatments used in first-generation algae-resistant granules, which were designed to completely replace common granules on early algae-resistant asphalt shingles.

Algae-resistant shingles using algae-resistant granules were first available during the early 1990s after 3M Industrial Mineral Products Division commercialized granules with heavily copper-loaded ceramic coating. The commercial introduction of a similar product from ISP Minerals quickly followed. Since then, at least two other roof granule suppliers—CertainTeed Corp., Valley Forge, Pa., and IKO, Brampton, Ontario—have commercialized similar algae-inhibiting granules.

Algae-resistant granules currently used make up a relatively small portion of the surfacing material embedded in the exposed parts of shingles. Based on information disclosed in patents, it is reasonable to say algae-resistant granule loading levels of 11 percent or less of shingle exposure surfacing by weight are used in currently available algae-resistant shingles. Asphalt shingle manufacturers do not disclose product composition details.

Present-day pigmented algae-resistant granules undergo multiple coating and firing steps during manufacture. The process is more time-, material- and energy-consuming than the process for common pigmented roof granules. As a result, algae-resistant granules command significantly higher prices.

The active ingredient in algae-resistant granules is cuprous oxide powder, known for its antimicrobial properties. It is included in the slurry used to produce the inner or base granule coating layer. The outer granule coating typically is heavily pigmented to obtain granule colors different from the inner coating's characteristic reddish-brown. It also is designed to control the long-term release of copper ions that prevent microbial growth on asphalt shingles.

Substantial cuprous oxide levels are necessary to supply the minimum ionized copper concentration required to effectively suppress algae growth on shingles.

The amount of algae-resistant granules in exposed shingle top surfacing is a key factor in achieving acceptable performance. An algae-resistant granule loading level that is too low will result in ineffective algae-retardant action, and shingles may develop stains. On the other hand, if the granule content is too high, the manufacturer incurs cost increases from overuse of granules.

Uniform distribution of algae-resistant granules in exposed shingle top surfacing is as important for performance as the loading level. The mechanism for the release of the antimicrobial agent has a limited effective radius. If algae-resistant granules are not distributed uniformly on an asphalt shingle, it is not equally protected and stains may develop in locations where the granules are scattered too far apart.

When moisture collects on asphalt shingle roof systems, algae-inhibiting copper ions gradually diffuse through the granule coating layers. As a result, over time, small quantities of the algae inhibitor are released from the granules. Rooftop moisture acts as a medium for the distribution of the algae inhibitor on roof surfaces.

According to asphalt roofing industry experts, current algae-resistant roof granule technology has been effective in preventing microbial staining of asphalt shingles in real-world weathering conditions for periods in excess of 10 years.

This technology is most effective in the presence of recurring surface moisture. Roof surfaces that are shaded or face away from the sun are more likely to be colonized by Gloeocapsa or related species because they take longer to dry out after a morning dew, for example, and receive less intense or indirect sunlight. North- and west-facing roof sections and those shaded by overhanging tree branches are a common setting in which pronounced microbial stains can readily develop.

Shingle color also has the potential to affect the spread of algae. A light-colored surface maintains a lower temperature than a similar dark surface. This slows evaporation, and the time window for damp conditions is extended. It has not been established whether this significantly influences colonization and growth of Gloeocapsa.

In arid regions, rooftop condensation is less frequent and blue-green algae growth is less likely. Current antimicrobial protection technology is less effective in dry-air environments because it has to rely on rainwater instead of dew. Where infrequent rainfalls are interspersed with extended dry periods, moisture does not remain on roofs for long. Because rainfall tends to rapidly clear away the algae inhibitor and roof surfaces dry more quickly, conditions optimal for the release of algae-inhibiting copper ions are not attained.

What's available

3M Industrial Mineral Products Division introduced its current algae-resistant system for asphalt shingles to the market in 1990 under the trade name Algae Block.™ It was available with a 10-year limited warranty against discoloration caused by microbial growth.

In 2003, the company began to market the same algae-resistant system under its well-known Scotchgard™ brand and extended the warranty length to 20 years. Currently, PABCO® Roofing Products, Tacoma, Wash., and Malarkey Roofing Products, Portland, Ore., offer the Scotchgard Algae Resistant Roofing System.

ISP Minerals' algae-resistant system for shingles is Copper Color Guard.™ ISP Minerals, as well as 3M Industrial Mineral Products Division, supply algae-resistant granules in a few colors.

GAF-Elk brand shingles, manufactured by GAF Materials, use the StainGuard® label. The remaining asphalt shingle manufacturers use the generic "AR" or similar label for their algae-resistant products.

There are a number of other North American roof granule suppliers. CertainTeed and IKO manufacture roof granules, including the algae-resistant variety, and supply their asphalt shingle operations internally.


Asphalt shingle manufacturers' product warranties treat resistance to discoloration from algae growth separately from other provisions contained in the warranties. Asphalt shingle algae resistance warranty coverage length is shorter than that for manufacturing defects. Manufacturers regularly review and update the language of asphalt shingle limited warranties. This process can materially alter the warranty terms from one revision to the next.

Asphalt shingle algae resistance warranty lengths currently available range from five to 20 years depending on the manufacturer and specific product. High-end asphalt shingles offer longer warranty coverage. Manufacturers commonly require the installation of their brand of algae-resistant hip and ridge shingles as a condition for obtaining algae resistance warranty coverage.

Under the terms of a typical asphalt shingle algae resistance warranty, the manufacturer reserves the right to clean the affected shingles rather than replace them. Two manufacturers, IKO and TAMKO,® Joplin, Mo., limit the remedies available under their warranty terms to the cost of labor for cleaning only the affected shingles up to the maximum rate specified in the warranty. Primarily, this is a cost-saving remedy for what is widely regarded as a cosmetic issue with asphalt shingles.

Asphalt shingle manufacturers do not consider algae stains harmful to shingle performance. See Figure 3 for a summary of asphalt shingle algae resistance warranties currently available in the U.S.

Figure 3: U.S. manufacturer asphalt shingle algae resistance warranties as of February 2009
Click on the image above to view the table as a PDF.

Product information

I asked suppliers and manufacturers about their procedures used to verify the effectiveness of algae-resistant systems. Atlas Roofing Corp., Meridian, Miss.; CertainTeed; GAF Materials; IKO; Malarkey Roofing Products; Owens Corning, Toledo, Ohio; and PABCO Roofing Products say they rely on observations collected during the course of long-term shingle weathering studies. TAMKO did not respond.

3M Industrial Mineral Products Division and ISP Minerals require algae-resistant system compliance certification for asphalt shingle manufacturers that use their respective system brands in finished products. They also require those finished products to meet specific minimum algae-resistant granule content levels. Asphalt shingle manufacturers verify compliance by submitting finished product samples for testing. Random audits of ongoing production also may be conducted at manufacturing plants. Products randomly obtained from retail also may be audited. In addition, there are provisions for sample submittal and testing for warranty claims.

3M Industrial Mineral Products Division and ISP Minerals compete for the business of asphalt shingle manufacturers. They are regarded as the largest granule suppliers to the roofing industry. They do not directly compete for roofing product end users because neither company distributes finished roofing products. (I would be remiss not to note that ISP Minerals and GAF Materials have common ownership though they are independently operated.)

Atlas Roofing, CertainTeed, GAF Materials and IKO do not rely on algae-resistant system supplier certification programs or supply granules internally and report they have internal verification procedures in place. Owens Corning declined to comment; TAMKO did not provide a response.

Asphalt shingle manufacturers disagree whether the same algae-resistant formulation should be used in different geographic markets for products that are otherwise indistinguishable.

Malarkey Roofing Products and PABCO Roofing Products, which both use the Scotchgard system, report the same system specifications are used in all markets. CertainTeed's algae-resistant formulations use internally sourced granules and are uniform across all geographic areas within any one of its product lines. Atlas Roofing stated that plant locations serving different markets are using formulations that have been found to work for their specific markets. GAF Materials, IKO and Owens Corning declined to comment. TAMKO did not provide a response.

I surveyed all known suppliers of algae-resistant products and systems about commercially available technologies for long-term algae resistance. All who responded confirmed that though there are several sources of algae-resistant systems for asphalt shingles, they all use similar copper-based technology.

"The copper granule formula currently appears to be the only commercially available algae-resistant technology for durable algae-resistant protection of asphalt shingles," says Husnu Kalkanoglu, CertainTeed's vice president of research and development. "New technologies that show promise are being developed for long-term algae-resistant protection. More time is needed to fully investigate them before commercializing."

High-end asphalt shingles currently available universally include algae resistance as a standard feature. Generally, architectural shingles receive algae-resistant labeling. Exceptions are found in markets with no demand for algae resistance. Availability of algae-resistant three-tab shingles is subject to local market preferences.

No agreement

Despite the progress made in algae-resistant asphalt shingle technology, there still is no recognized standard to help gauge given products' effectiveness.

3M has cooperated with Owens Corning, a member of the Asphalt Roofing Manufacturers Association (ARMA), to develop a laboratory method for quantifying asphalt shingles' algae resistance.

The method consists of two parts. First, during a period of a few months, Gloeocapsa colonies are grown under standard conditions on panels cut from asphalt shingles. The light, humidity and nutrient supply are optimized to accelerate colonization. The method employs techniques that are similar to those used in existing ASTM International test methods for evaluation of a building's interior materials' resistance to microbial growth. Next, each panel is evaluated for the extent of visible microbial growth and a numeric score is assigned. No pass/fail criteria are defined in the method, and currently, no further information is publicly available.

This work was presented to ARMA for review several years ago. A task force was set up, but the matter eventually was dropped because ARMA members could not reach an agreement.

In 2007, the same proposed method was brought before the ASTM Committee D08 on Roofing and Waterproofing. It currently is undergoing review by a task force of roofing industry experts.

The interested parties are split about whether the proposed method is appropriate to become a recognized standard.

Those arguing against a standard focus on two key flaws. Most significantly, the proposed method may not be able to distinguish the action of nondurable algae-resistant treatments, which can lose efficacy after as little as a few months of weathering, from that of systems designed for long-term delivery of algae inhibitors. As such, the method could be exploited to misrepresent product performance and potentially cause confusion.

Critics also assert the proposed method does not model the varied environmental conditions shingles encounter through-out North America, and, therefore, it cannot accurately predict real-life shingle performance.

Lou Hahn, GAF Materials' vice president of research and development for GAF-Elk, points out that "the proposed practice is limited in practicality and has the potential for misuse."

"It is limited to one species of algae, Gloeocapsa, which, though one of the major species, is not the only one, particularly in some geographic areas," he continues. "It only works for one [copper release] mechanism type while others are possible. Attempts to standardize around the behavior of a biological system, particularly with the variability of materials and environments to which it is exposed, can be dangerously misleading."

On the other side of the argument are those who see the proposed method as a practical necessity and the next logical step for roof granule industry researchers, who so far have had to patiently wait for years to gauge effectiveness of algae-resistant asphalt roofing products.

"I find the method is useful for evaluating current copper granule technology, particularly because it can be tweaked to model specific weathering conditions," Kalkanoglu says. "This is not to say that it's perfect. We should be open to ideas for improving it. That's how the ASTM process works."

Stay informed

Unless you happen to shop for asphalt shingles in places such as Idaho or Arizona, labels advertising algae resistance are almost ubiquitous on asphalt shingle packaging. Algae-resistant system suppliers and shingle manufacturers appear to have found a favorite solution to the staining issue and have been providing versions of it for nearly two decades. During this time, they have accumulated a considerable amount of product performance data, yet no recognized standard has emerged. The only clues to a product's efficacy are found in its warranty terms.

With regard to algae-resistant shingles, NRCA holds the position that roofing customers should focus their purchasing decisions primarily on objective, comparative evidence of proven performance in service, not warranty time frames.

Maciek Rupar is an NRCA director of technical services.


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